Combustion of coal in power utility fired boilers produces two types of waste products: fly ash, which includes ash particles that are small enough to be entrained in flue gas, and bottom ash, which includes larger ash particles that overcome drag, and drop to the bottom of the boiler. A number of systems have been developed to transport the fly ash to a storage silo.
However, existing fly ash transport systems function at low capacity, consume large amounts of energy, have high ash consolidation times in the hoppers (causing the ash to clump), convey the ash at high velocity (causing transport pipe erosion), and require larger transport pipes.
An electrostatic precipitator or a bag house system typically collects the fly ash, after which storage hoppers underneath the precipitator or bag house system collect the fly ash. The fly ash is then transported from the storage hoppers via the transport piping to a storage silo. The hoppers are typically pyramid shaped vessels designed to maximize space and volume. Unfortunately, pyramid hoppers are inefficient and erratic in discharging fly ash in comparison to conical hoppers because of friction between adjacent walls.
Fly ash is discharged from the hoppers and transported pneumatically, either via a vacuum or pressure system, to a silo which is several hundred feet away. In either case, the fly ash can be transported by way of dilute phase transport or dense phase transport. In dilute phase transport, the pickup velocity, which is the air velocity based on total pipe area, is typically above 3,500 feet per minute. Thus the solids are mostly suspended during conveyance. By comparison, in dense phase transport, the pickup velocity is nominally below 2,000 feet per minute, which yields high solid transport rate but with the solid being only partially suspended. This type of transport is used for higher conveyance rate and lower pipe erosion.
In a vacuum system, a vacuum pump sucks transport air and fly ash from the hoppers and then through transport piping. The air/ash mixture discharges into a collection tank having bag filters to clean air prior to discharge into the atmosphere via the vacuum producer. System operating pressure is near 18 inHg vacuum and is typically measured upstream of the vacuum producer.
In pressure type pneumatic conveying systems, pressurized air transports fly ash. Vessels (i.e. airlocks) underneath hoppers feed solids into a pressurized transport line. The hoppers are typically at or slightly below atmospheric pressure. Each airlock has a volume tank, and an inlet valve, an outlet valve, and an equalizer valve each attached to the volume tank. An equalizer valve is three-way valve used to equalize the pressure between the airlock and the hopper during fill or between the airlock and the transport line during discharge.
To allow the solids to be transferred by gravity from the storage hopper through the airlock and to the pressurized transport line, an airlock's three valves are opened and closed in a programmed sequence. Since the airlock tank's volume usually is considerably less than the hopper's volume, the airlock feeds the transport line in multiple batches before the hopper is empty.
In some pressure transport systems, the airlock sequence logic opens only one airlock at a time into the transport line. As the airlock outlet valve opens and the solids are fed into the transport line, the pressure rises as the transport line reacts to resistance created by the solids. The greater the quantity of solids being conveyed, the higher the system pressure will become. Dumping one airlock at a time can be characterized by a pressure rise when the airlock outlet valve first opens, followed by a steady falloff in pressure as the airlock empties. This is inefficient because when the pressure falls, the transport velocity rises causing higher erosion of the pipe wall, and the system is not moving solids to its maximum extent. For example, the system often operates at or near design pressure only 25-50% of the time. This is largely due to the inability of the hopper, especially the pyramid type but also the conical type, to discharge enough material to keep the airlock full. Also, certain fly ash material discharges poorly from both the airlock and the hopper. Slow discharge starves the transport line in a single airlock dump system.
Dense phase type transport systems with simultaneous multi vessel discharge empty all airlocks in a row at once. However, in these systems the airlocks do not have bottom valves. Instead, the transport vessel has only one valve that separates the transport piping from the hopper. Thus the fly ash is dropped directly into the transport piping using multiple transport vessels, after which pressurized air is introduced at multiple locations to convey the material in slugs.